Synthesis and Color Evaluation of Ta5+-Doped Bi2O3

Ta 5+ -doped Bi 2 O 3 solid solutions, (Bi 1- x Ta x ) 2 O 3+2 x (0 ≤ x ≤ 0.20), were synthesized by a conventional solid-state reaction method. The crystal structure, optical properties, and color of the pigments were characterized. The (Bi 1- x Ta x ) 2 O 3+2 x ( x = 0, 0.03, 0.04, 0.20) samples were obtained in a single-phase form, and the crystal structure depended on the Ta content; they were monoclinic α-phase ( x = 0), tetragonal β-phase ( x = 0.03, 0.04), and cubic δ-phase ( x = 0.20). Among them, (Bi 0.97 Ta 0.03 ) 2 O 3.06 strongly absorbed visible light at the wavelength shorter than 480 nm and exhibited the most vivid orange color. The redness value ( a *) was comparable to that of commercial orange pigments, and the yellowness value ( b *) was slightly larger. Although it is necessary to improve chemical stability, the (Bi 0.97 Ta 0.03 ) 2 O 3.06 pigment has potential for an environmentally friendly inorganic orange pigment.